In order to better characterize the invention and the problem it solves, five categories of existing polyamide materials will be mentioned. The term “polyamide materials” is understood to mean compositions based on polyamides, copolyamides and alloys of polyamides or based on polyamides.
(1) Impact-Modified Polyamide Materials (High Impact PA)
These are alloys of polyamide with a minor amount of elastomer, typically in the vicinity of 20% by weight. The polyamide is typically a semicrystalline polyamide. These alloys have the advantage of a very good impact strength, much improved with respect to polyamide alone, typically three times or more better. They also have good chemical strength and satisfactory resistance to distortion under heat (60° C.). They have the disadvantage of being opaque, which can be a problem for decorative components.
(2) Transparent Amorphous Polyamide Materials (TR amPA)
These are materials which are transparent, which are amorphous or not very semicrystalline, which are rigid (flexural modulus ISO>1300 MPa) and which do not distort under heat, at 60° C., as they usually have a glass transition temperature Tg of greater than 75° C. However, they are rather unresistant to impacts, exhibiting a much lower notched Charpy ISO impact in comparison with impact-modified polyamides (typically 5 times less), and their chemical resistance is not excellent, typically due to their amorphous nature. There also exists (but these are materials less frequently encountered) transparent semicrystalline (or microcrystalline) polyamides, these materials also being fairly rigid and having a flexural modulus ISO>1000 MPa.
(3) Polyether-Block-Amide and Copolymers Comprising Ether and Amide Units (PEBA)
These are copolyamides based on ether units and on amide units: polyetheramides and in particular polyether-block-amides (PEBAs). These are very flexible impact-resistant materials but with a fairly low transparency (45 to 65% of light transmission at 560 nm for a thickness of 2 mm), just as for their polyamide homologues without ether units. The Pebax products from Arkema are an illustration thereof.
(4) Semicrystalline Polyamides (PA)
These are typically linear aliphatic polyamides. Their crystallinity is reflected by the presence of spherolites, the size of which is sufficiently great for the material not to be highly transparent (light transmission of less than 75% at 560 nm). PA11, PA12 and PA6.12 are an illustration thereof.
(5) Transparent Semicrystalline Polyamides (TR scPA)
These are more specifically microcrystalline polyamides where the size of the spherolites is sufficiently small to retain the transparency.
The various properties of the five categories of polyamides which have just been indicated have been summarized in Table A below.
Definitions of Table A:
    (a) Transparency: It is characterized by the measurement of transmission at 560 nm through a polished sheet with a thickness of 2 mm.    (b) Impact/breaking strength: It is characterized by a rapid bending test or by a notched Charpy impact ISO179.    (c) Flexibility: It is characterized by the flexural modulus ISO178.    (d) Temperature stability: Ability of the polyamide not to be distorted if it is placed in a hot atmosphere, at approximately 60° C., and under the effect of a relatively great weight. For an amorphous or essentially amorphous polymer, the temperature stability becomes better as the Tg (glass transition temperature) increases and is greater than 75° C. For an essentially semicrystalline polymer, the temperature stability becomes better as the M.p. (melting point) increases and is greater than 100° C. and in particular as the enthalpy of fusion increases, this enthalpy being the reflection of the degree of crystallinity.    (e) Chemical resistance: Ability of the polyamide not to be damaged (matifying, cracking, splitting, breaking) on contact with a chemical (alcohol, and the like) and in particular if it is placed under stress, that is to say “stress cracking”.    (f) Elastic fatigue: Ability of the polyamide to be bent a large number of times without breaking, elastic rebound, for example “Ross-Flex” test.    (g) Processing, ability to be injection moulded: Ability of the polyamide to be easily processed by an injection-moulding process (short cycle time, easy removal from the mould, undistorted component).
The aim of the invention is to find novel transparent compositions which are impact resistant, which are not too rigid and even up to very flexible, which behave well towards or are resistant to distortion under heat (60° C.) and/or which have good chemical resistance. The ability to resist alternating bending (fatigue) and the ability to be easily processed by injection moulding are also desired qualities. In other words, the aim has been to find a composition combining most of, or at least a larger number of, the advantages of the first three categories above (high-impact PA, TR amPA, PEBA).
The PEBA copolymers belong to the specific category of the polyetheresteramides when they result from the copolycondensation of polyamide sequences comprising reactive carboxyl ends with polyether sequences comprising reactive ends which are polyether polyols (polyether diols), the bonds between the polyamide blocks and the flexible polyether blocks being ester bonds, or alternatively to the category of the polyetheramides when the polyether sequences comprise amine ends.
Various PEBAs are known for their physical properties, such as their flexibility, their impact strength or their ease of processing by injection moulding.
The improvement in the transparency of PEBAs has already formed the subject of various research studies.
Blends between semicrystalline PEBAs have been produced but the improvement in the transparency obtained remains modest and far below 75% of transmission (at 560 nm, through a polished sheet with a thickness of 2 mm). In the case of a blend of a semicrystalline PEBA with a Tg of less than 50° C. of block-PA12/block-PTMG type with another semicrystalline PEBA with a Tg of less than 50° C. of block-PA11/block-PTMG type, the improvement in transparency reaches at best a transmission of 49%, at 560 nm over a thickness of 2 mm, which corresponds to an object which is still very markedly hazy to the eye. Blends of this type can only really be suitable for thin objects, where the haze will thus be less noticeable.
Generally, known copolymers comprising ether and amide units are composed of semicrystalline and linear aliphatic polyamide sequences (for example the “Pebax” products from Arkema).
The Applicant Company has discovered, surprisingly, that if, on the contrary, use is made of polyamide monomers of cycloaliphatic nature (and thus of nonlinear aliphatic nature), if they are copolymerized with flexible polyethers, which gives an amorphous copolymer (A), and if, subsequently, the said copolymer obtained is blended with another polyamide (B), in particular with a known semicrystalline PEBA (or other known copolymers comprising ether and amide units), transparent compositions with much improved properties are obtained. In particular, materials which are resistant to distortion under heat at 60° C. are obtained. These compositions have an improved impact strength and an improved flexibility. These compositions combine the qualities of the amorphous constituent (A), in particular its Tg, which surprisingly remains virtually unchanged, and the qualities of the semicrystalline PEBA, in particular its melting point and its enthalpy of fusion, which surprisingly remain virtually unchanged. These compositions exhibit virtually none of the failings of each of their constituents; in particular, they do not exhibit the poorer alternating flexural fatigue performance (“Rossflex”) of (A) and the low transparency of (B).
In the prior state of the art, transparent blends (or alloys) of polymers were tried in order to improve the abovementioned properties. For example, EP 550 308 and EP 725 101 disclose alloys of transparent amorphous polyamide combined with a nontransparent semicrystalline polyamide, the combination giving a transparent and less rigid material. However, this material remains highly rigid (>1200 MPa of ISO flexural modulus) and has modest impact qualities (ISO notched Charpy impact of the order of 7 kJ/m2, to be compared with 50 or more for a high-impact polyamide). Furthermore, its Tg has declined strongly if it is compared with that of the amorphous component alone. For example, Cristamid MS1100 from Arkema, a blend of an amorphous polyamide with a Tg of 170° C. and of 30% of semicrystalline PA12, has a Tg of 110° C. What is more, this blend almost never any more has the melting point or the enthalpy of fusion of the component PA12, even with respect to the amount of PA12 present. Another example of this type of material is Grilamid TR90LX (Ems), with a Tg far below that of its predominant component, Grilamid TR90.
Another known possibility for improving consists in blending, with the amorphous polyamide, a semicrystalline copolymer based on amide units and on ether units. However, the level of transparency obtained is much poorer than in the preceding case and it is necessary to strongly heat the blend in order to obtain an acceptable transparency. Moreover, these blends still have the disadvantage of remaining fairly rigid, in any case much more rigid than the blend of the present invention. These blends also have the disadvantage of an alternating flexural fatigue (“Rossflex”) significantly inferior to that of the blends of the present invention.
If use is made, instead of the transparent amorphous polymer, of the said copolymer (A) and if it is combined with a semicrystalline polyamide, in particular with a semicrystalline copolyamide comprising ether units and comprising amide units (for example, a polyesteretheramide or a polyether-block-amide PEBA), then not only is a material obtained which is transparent and resistant to distortion under heat at 60° C. but also a material which is significantly less rigid and more impact resistant. This blend also has an improved chemical resistance in comparison with the said copolymer (A) alone and an improved elastic fatigue strength.
The problem which consists in finding novel transparent compositions which are impact resistant, which are not too rigid and even up to very flexible, which are easy to process, which have good resistance to distortion under heat (60° C.) and/or good chemical resistance and/or which have good fatigue strength can thus be solved by the use of the copolymer (A) by combining the latter in the form of blends or of alloys with other polyamides, advantageously semicrystalline polyamides, or more advantageously still their form copolymerized with ether units, in particular PEBAs.